1. Field of the invention
This invention relates to a method for operating the reactor for producing polyolefins through a vapor phase polymerization method. More particularly, the invention relates to a method for restarting the operation after the interruption of reaction in the preparation of polyolefins through vapor phase polymerization.
2. Description of Prior Art
The vapor phase polymerization method of olefins to prepare a polyolefin is widely employed for the reason that its production cost is low. The system for the vapor phase polymerization is exemplified by a fluidized bed system or a stirred bed system. (cf. British Patent Nos. 1,248,951, 1,248,952, and 1,248,953; U.S. Pat. No. 3,971,768).
In the vapor phase polymerization to prepare polyolefins, various kinds of serious situations happen to occur to interrupt the operation of the reactor due to several troubles or remedy work of equipment. For example, troubles are often caused to occur in the steps of powder treatment, pelletizing and blending subsequent to the polymerization step, or in the cases that temporary storage tanks are filled up with produced polymer particles or a gas blower for recycling is out of order. In these troubles, the polymerization process is stopped not completely but temporarily and, after the remedy of a trouble, the operation is restarted without delay.
For stopping the reaction, a deactivator is sometimes introduced into a reaction system. However, under some other operation conditions, the deactivator is not used so as to avoid undesirable influences that are caused by the deactivator. The term "deactivator" used herein is intended to mean an agent that interrupts a polymerization reaction proceeding at some stage.
Without the use of deactivator, the reaction can be temporarily stopped and started again by the following methods.
(1) The feeds of a solid catalyst component and an organoaluminum compound is discontinued and the feeds of gases including olefin are reduced in proportion to the lowering of the rate of reaction. After the rate of reaction is lowered to a certain level such as to a half or a third of the regular reaction rate, all the feeds of reactant gases are stopped.
(2) The feeds of all gases are stopped simultaneously with the stopping of feeds of solid catalyst component and organoaluminum compound and the pressure and temperature are lowered.
In any stopping process, the pressure and temperature are lowered and gases in the reaction system are purged with an inert gas, and the polyolefin particles remaining in the reaction vessel are discharged.
The reason for the discharge of polyolefin particles is as follows. The concentrations of the solid catalyst component and organoaluminum compound in the remained polymer particles vary in each stopping operation. If the operation is restarted with the remained polymer as it stands, the conditions to start reaction are not settled and vigorous reaction is sometimes caused to occur in the initial stage of reaction or, to the contrary, the starting of reaction takes many hours. In addition, as described later on, the trouble due to the formation of sheet-like polymer is liable to occur.
Accordingly, the restarting may be carried out after feeding the reaction vessel with new polyolefin particles.
The method for emergency stop by introducing a deactivator into a reaction system and its restarting are as follows.
The polymerization reaction is stopped by feeding a deactivator such as carbon monoxide gas or carbon dioxide gas into a reaction vessel (cf. EP-A No. 136029), which is followed by the purging of gases in the reaction system with an inert gas such as nitrogen. After that, the polyolefin particles remaining in the reaction vessel is discharged.
The mechanism to stop the reaction using a deactivator is such that the reaction between a deactivator and a catalyst or co-catalyst is firstly caused to occur and, as a result, the catalyst loses its function to stop the reaction. The reaction between a deactivator and a catalyst or co-catalyst is analyzed to some extent, however, the influences of its reaction products on the polymerization reaction has not been clarified sufficiently. In addition, there is similar apprehension when a deactivator is allowed to remain in the reaction system.
Accordingly, it has been a usual practice that the reaction is stopped by a deactivator as described above, and when the reaction is started again, not only a deactivator and gases including reactant gases but also polyolefin particles remaining in the reaction system are all discharged substantially. After that, polyolefin particles are newly fed into the reaction system and reactant gases and catalysts are then supplied so as to restart the operation.
In this method as described above, the preparation is usually restarted by the following method. The polyolefin particles remaining in a reaction vessel are discharged and the polyolefin particles produced in a regular state or those produced in a separate process are introduced into the reaction vessel, the space within the reaction system is subjected to inert gas purging, and the operation of reaction is started again. In this method, however, all the contents in the reaction system are changed in order to restart, which is not different from the operation of newly starting a reaction.
Furthermore, the fact that the troubles due to the sheet-like polymer is liable to occur in the initial stage of vapor phase polymerization of polyolefin, has already been disclosed (EP-A Nos. 224479, 313087, 315192, and 366823). Accordingly, various troubles due to the formation of sheet-like polymer are caused to occur also in the initial stage of the restarting operation.
As described above, when the vapor phase polymerization of polyolefins is urgently stopped with or without a deactivator and the operation of the reactor is started again, there have been several disadvantages as follows in the conventional method:
(1) The sheet-like polymer is liable to be formed in the initial period of the operation of reactor for polyolefin. The stopping of operation is unavoidable due to the blocking of pipings and valves with the sheet-like polymer.
(2) When the operation is restarted, the feed quantity of catalysts is gradually increased to raise the rate of formation of polyolefin. Accordingly, temporary non-regular conditions are-continued during which period a wide specification material is produced.
(3) The reactor must be exposed to the air when the polyolefin particles are discharged or new polyolefin particles are fed. In such an operation, impurities such as moisture and oxygen are liable to be introduced into the reaction system. Therefore, the polymerization reaction is hardly started in the restarting operation necessitating a long time to attain regular operation. This phenomenon is severe when the reaction system is exposed to the air for a long period of time.
Accordingly, it is eagerly wanted to improve the restarting operation after the temporary stopping of the reactor for producing polyolefin.